In olefin epoxidation, an olefin is reacted with oxygen to form an olefin epoxide using a catalyst comprising a silver component, usually with one or more elements (i.e., promoters) deposited on a carrier. Catalyst performance is characterized on the basis of selectivity, activity and stability. The selectivity is the molar fraction of the converted olefin yielding the desired olefin oxide. The fraction of olefin reacted normally decreases with time and to maintain a constant product level the temperature of the reaction is increased. However this adversely affects the selectivity of the conversion to the desired product. Thus, the longer the selectivity can be maintained at a high level and at an acceptably low temperature, the longer the catalyst charge can be kept in the reactor and the more product is obtained. Quite modest improvements in selectivity and the maintenance of selectivity over longer time yield huge dividends in terms of process efficiency.
Beside the chemical composition of a supported silver-based epoxidation catalyst, the physical characteristics of the finished catalyst as well the support have been an integral part of catalyst development. Generally, the silver-based catalyst support shows a characteristic pore volume and pore size distribution. Furthermore, the surface area and the water absorption are well-known characteristics for such catalyst supports. It has now been found that the physical characteristics of the finished catalyst and the impact of these characteristics on the catalyst performance are more complicated than heretofore believed, especially if the catalyst is promoted with rhenium. In addition to the surface area, the pore volume and the pore size distribution, the pattern of the pore size distribution, especially the number and the specific characteristics of different modes, has been found to have a significant positive impact on the catalyst selectivity.
A number of patents describe preparation and selection of different, preferential carriers for ethylene epoxidation catalysts. For example, U.S. Pat. No. 4,242,235 discloses low surface area, less than 10 m2/g, carriers. This prior art carrier has a porosity of 60% and is bimodal with mean pore diameters in the range of 1-5 micron and 60-200 microns. Each of the ranges disclosed in the '235 patent represents at least 35% and at most 65% of the total porosity.
U.S. Pat. No. 4,226,782 describes a carrier having a surface area in the range from 0.05 to 10 m2/g, a porosity from 30-80%, and pores of 0.1 to 20 microns. U.S. Pat. No. 5,266,548 describes a method of making alpha-alumina from 95% high purity alumina. Preferably, the carrier disclosed in the '548 patent has a porosity of from 0.2 ml/g to 0.6 ml/g, and a surface area from 0.2 m2/g to 10 m2/g. Additionally, the '548 patent discloses that the average pore size of the carrier is from 0.1 microns to 100 microns, preferably in the range from 0.1-10 micron, and more preferably from 0.2-5 micron.
U.S. Pat. No. 5,380,697 describes the preparation of an alumina carrier from alpha-alumina particles having a median grain size of from 0.4 micron to 4 micron and a sol-gel. The carriers made in the '697 patent typically have a pore volume (i.e., water absorption) ranging from about 0.2 cc/g to about 0.6 cc/g, preferably from about 0.3 cc/g to about 0.5 cc/g and a surface area ranging from about 0.15 m2/g to about 3 m2/g, preferably from about 0.3 m2/g to about 2 m2/g.
U.S. Pat. No. 5,597,773 discloses a large number of refractory carriers, with carriers made of alpha-alumina being highly preferred. In the '773 patent, preference is given to the use of alpha-alumina carriers having a specific surface area, as measured by a BET method, of from about 0.03 m2/g to about 10 m2/g, preferably from about 0.05 m2/g to about 5 m2/g, more preferably from about 0.1 m2/g to about 3 m2/g, and a water pore volume, as measured by conventional water absorption techniques, from about 0.1 cc/g to about 0.75 cc/g by volume, preferably from about 0.3 cc/g to about 0.5 cc/g.
In U.S. Pat. No. 5,929,259, a carrier for ethylene oxide catalysts having a porosity at least 50% and more desirably from about 60% to about 75% is disclosed. The surface area of the carrier disclosed in the '259 patent is preferably in the range 0.4-5 m2/g, and more preferably 0.6-1.2 m2/g.
U.S. Pat. No. 6,831,037 describes a technique for the preparation of an alumina carrier. The carrier is 95% alpha-alumina having a surface area in the range from 1.0-2.6 m2/g, and a water absorption from 35-55%. This prior art carrier is advantageous for ethylene oxide catalyst preparation when at least 70% of pore volume is in the range from 0.2-10 microns, which provide at least 0.27 ml/g of total pore volume. Pores with diameters less than 0.2 micron provide 0-10% of pore volume, and pores with diameters more than 10 micron provide less than 20% of total pore volume.
U.S. Patent Application Publication No. 2004/0110973 A1 discloses a method for the preparation of an alumina carrier from two alpha-alumina particulates. This prior art carrier has 75% of pores with a diameter in the range from 0.2-10 micron, less than 20% of pores with diameter of more than 10 microns, and less than 10% of pores with diameter of less than 0.2 micron. Water absorption of this prior art carrier is at least 0.35 ml/g, and the surface area is in the range from 0.6-5 m2/g.
U.S. Patent Application Publication No. 2005/0096219 A1 found advantages to provide a carrier which has a minimum of very large pores, greater than 10 micron, and water absorption of 35 to 55% and a surface area of at least 1.0 m2/g. A method of making such a carrier is also described in the '219 publication.
As described above a catalyst for ethylene epoxidation requires a carrier with specific physical properties. It would be desirable to improve the catalytic selectivity, activity and stability of the catalysts by improving carrier characteristics. It has been unexpectedly found by the applicants of the present application that the pore distribution of the carrier, and particularly the absolute value of pore volume from the pores of a different diameter range, define performance of an ethylene oxide catalyst, particularly selectivity and stability.